At present, the fabrication of silicon solar cells involves the use of self-supporting crystalline or polycrystalline wafers. However, there are a number of advantages which can be obtained by the fabrication of silicon solar cells in the form of thin silicon films on a supporting layer. Accordingly, there is an increasing amount of interest with respect to commercially applicable methods of fabricating silicon solar cells in thin-film form.
There is also an increasing interest in depositing silicon of electronic quality over large areas for consumer applications such as liquid crystal displays for application as television screens. One such deposition technique is carried out by first dissolving silicon in a molten metal so that the melt is saturated with silicon and then cooling. Upon cooling, the amount of silicon which can be dissolved in the melt decreases. The excess silicon can be made to precipitate, out onto a substrate, at a controlled rate.
At present, tin (Sn) is the most commonly used metal for dissolving silicon for subsequent deposition. Tin is desirable in that it is electrically quite inert in silicon. Accordingly, the unavoidable incorporation of tin in the deposited silicon layer does not detract from the layer's electronic properties. However, tin is not desirable in that high temperatures (greater than 900.degree. C.) are required to dissolve much silicon in tin. The high deposition temperature severely limits the choice of substrate material due to thermal mismatch considerations. A further limitation upon substrate choice as well as upon the choice of processing conditions and cleanliness requirements is provided by the much higher prospects for contamination of the silicon layer by other impurities at high temperatures.
Gold forms a eutectic with silicon and has a unique ability to dissolve large amounts of silicon at low temperatures. The eutectic composition consists of about 18% silicon (atomic basis) with a corresponding eutectic temperature of about 363.degree. C. This means that at any temperature higher than this, a molten solution of silicon in gold can be formed with the silicon content being at least as high as at the eutectic. Although gold is very detrimental to the electronic properties of the silicon layer, (when incorporated into this layer in even very small quantities) the low deposition temperature means that virtually no gold is incorporated into the lattice structure of the silicon layer. Similarly, only small quantities of other impurities will be incorporated at such low temperatures. Accordingly, processing can be carried out using relaxed cleanliness requirements for (1) the substrate material; and (2) the deposition equipment. In addition, the purity requirements of the substrate and the solutions employed can be reduced. The low deposition temperatures also reduce thermal mismatch problems between the deposited silicon layer and the substrate.
The use of gold in silicon deposition-processing is not desirable because of the high solubility of silicon in gold at the eutectic. Large amounts of silicon required to form a melt at low temperatures giving rise to difficulties in controlling the precipitation rate of silicon from the solution upon cooling. This reduces the crystallographic quality of the deposited film and also gives rise to the possibility of desirable macroscopic gold inclusions in the deposited film.